The invention is based on an electrical system particularly a system for electrical devices in a car.
1. Description of the Prior Art
A car radio in which the disconnection of the operation voltage activates an antitheft device is already known from the user's manual of the Blaupunkt car radio ACM 5450.
2. Advantages of the Invention
The electrical device according to the invention with the features of the main claim offers the advantages that, by uncomplicated means and in a defined time and manner, it is ensured that a code is activated rapidly and reliably after a voltage interruption even while the electrical device is still being removed.
On the basis of the features presented in the subclaims, advantageous further embodiments of, and improvements to, the electrical device specified in the main claim are possible.
Advantageous use can be made of the residual charge remaining in any case in the first electrical device. In this way, there is no need for any significant amount of extra effort to produce the voltage difference.
Advantageous to use a voltage divider, a diode, and a switch to bring about the defined initiation of the security procedure in the control unit. A simple and uncomplicated security circuit is realized, which is also very low in cost.
Advantageous to insert a second capacitor between the control input of the switch and the first reference potential, the capacitance of the second capacitor being smaller than the capacitance of the first capacitor. As a result, brief voltage interruptions, inductive interference, and voltage peaks are prevented from actuating the switch and thus from activating the security procedure in the control unit. It is thus ensured as far as possible that the security procedure will be activated in the control unit only when the voltage is interrupted during a theft.
It is advantageous that, after a voltage interruption, the power supplied to the control unit comes from the residual charge of the first capacitor. As a result, the control unit is still supplied with energy even after the voltage has been interrupted by a theft, which means in turn that the security procedure can still be activated.
It is advantageous that the security procedure is activated after a certain delay, which can be selected by means of a timer acting in concert with the control unit. This again prevents a brief voltage interruption attributable to a disturbance from activating the security procedure in the control unit.
It is advantageous that the electronic lock can be deactivated by entering a code word on a user input component of the first electrical device, the input component being connected to the control unit. As a result, in the event that a voltage interruption, possibly undesired, has led to the activation of the security procedure in the control unit, the user can easily restore the functional status of the electrical device after the voltage supply has been restored.
It is advantageous for the supply voltage or a variable which can be derived from the supply voltage to be measured in the first electrical device, for the measurement result to be sent to the control unit, and for an additional condition to be imposed for the activation of the security procedure in the control unit, namely, that the measured supply voltage or the variable derived from it must fall below a predetermined value in the first electrical device. This provides additional improvement to the protection of the security circuit against unintentional activation of the security procedure.
It is advantageous that it is possible to connect a resistor in parallel to the third capacitor, at which at least a portion of the supply voltage is present, so that the third capacitor can be discharged rapidly when the supply voltage is cut off. In this way, the goal is achieved that the corresponding portion of the supply voltage falls more rapidly than the operating voltage. After the control unit has detected that the corresponding portion of the supply voltage has dropped below the predetermined value, therefore, it still has enough time to activate the security procedure. Leakage currents during the time that the supply voltage is connected can be avoided by connecting the resistor only when the supply voltage is cut off.